For many years, naturally-occurring and induced gamma radiation has been measured to differentiate between different lithologies and/or densities and porosities of subterranean geological formations. In particular, naturally-occurring gamma radiation has been measured within wellbores to derive valuable information regarding the possible location of formations containing desired natural resources (e.g., oil and natural gas, etc.) and induced gamma radiation has been measured within wellbores to derive formation density and fluid-filled porosity.
FIG. 1 illustrates the use of a gamma radiation detector 200 used for obtaining these types of measurements during a well drilling operation. As is well known, natural resources such as oil and natural gas are extracted through a conduit (i.e., a wellbore 110) drilled into a formation 101 using a drill bit 108 located at the end of a drill string 106. A rotary drilling rig 120 supports the weight of the drill string 106 and imparts a rotational force to the drill string 106 to cause the drill bit 108 to create the wellbore 110.
The gamma radiation detector 200 is incorporated into a detector tool 102, which forms a housing for one or more detectors 200 and which is attached (e.g., by threads or collars) to the drill string 106, preferably just above the drill bit 108. The detector tool 102 may also be attached directly to the drill bit 108, although another section of drill string 106 may also intervene. As explained further below, the detector tool 102, and hence its detector 200, are exposed to and measure gamma radiation (either induced or naturally-occurring) emitted by the formation 101 traversed by the wellbore 110 as the wellbore 110 is drilled. These measurements, in conjunction with the known depth of the detector tool 102/detector 200 at the time the measurements are recorded, may provide information about the types of formations that are traversed by the wellbore 110. For example, higher levels of gamma radiation at a particular depth may indicate a shale formation 112, which are known to contain higher concentrations of naturally-occurring radioactive materials that emit gamma radiation.
FIGS. 2A and 2B show horizontal (section A-A) and vertical (section B-B) cross sectional views of components within a particular detector tool 102A having a single gamma detector 200. The detector 200 is placed within a pocket 220 formed in the periphery of the detector tool 102A to minimize the distance between the detector 200 and the gamma radiation source, i.e., the formation 101 traversed by the wellbore 110.
The detector 200 includes a scintillation crystal 202 and a light sensor 204, preferably a photomultiplier tube (PMT). As will be described in greater detail below, gamma radiation may interact with the scintillation crystal 202 through various mechanisms to cause the scintillation crystal 202 to emit visible or ultraviolet light that is converted to measurable electrical pulses by the light sensor 204. Electronic circuitry 250 controls the light sensor, and also measures the pulses, as explained further below. Electronic circuitry 250 is shown in FIG. 2B in the form of an electronics module connected to the light sensor 204 portion of the detector 200, although integration of the electronics with the detector 200 and the detector tool 102 can occur in different manners. Spaces within the pocket 220 not occupied by the detector 200 may be filled with a material such as RTV silicone or epoxy. A drilling fluid conduit 210 for conveying drilling fluid from the surface to the drill bit 108 is in the example of FIGS. 2A and 2B displaced from the central longitudinal axis of the detector tool 102A to accommodate the placement of the detector 200 in the detector tool 102A. However, such displacement is not necessary if the detector 200 is small enough.
FIGS. 2C and 2D show horizontal (section A-A) and vertical (section C-C) cross sectional views of components within a different example of a detector tool 102B having three gamma detectors 200, although other plural numbers of detectors could also be used. The detector tool 102B includes the same types of components as the detector tool 102A, and is labeled with the same reference numbers. In the detector tool 102B, the detectors 200 are arranged in pockets 220 that are equally spaced around the periphery, i.e., at 120 degree intervals. The drilling fluid conduit 210 in the detector tool 102B is positioned between the detectors 200 along the central longitudinal axis of the detector tool 102B. The gamma radiation measured by each of the detectors 200 in detector tool 102B can be combined and processed to generate measurements that may be more precise than those produced by detector tool 102A having only a single detector 200. However, the detector tool 102B may have a larger diameter than the detector tool 102A, restricting its use in smaller-diameter wellbores 110.
FIGS. 3A and 3B show horizontal (section A-A) and vertical (section B-B) cross sectional views of components within an induced gamma radiation detector tool 102C that includes a gamma source 302 located within a housing 304. The gamma source 302 emits gamma radiation that passes through material between the tool 102C and the wellbore wall and interacts with the formation adjacent to the tool 102C, which results in induced gamma radiation. A portion of the induced gamma radiation is scattered back into the wellbore 110 where it is detected by one of two detectors 200A/B in the tool 102C. The gamma source 302, such as Cesium-137 or Cobalt-60, is selected such that the primary mode of interaction of the emitted radiation with material it traverses is Compton scattering, which is described below. Compton scattering is related to the electron density of the material in which the interaction occurs, and, electron density is, in turn, related to the bulk density of the material. Therefore, the gamma radiation detected by the detectors 200 can be related to the bulk density of the formation.
Because the source gamma radiation interacts not only with the formation but also with intervening material such as drilling fluid between the tool 102C and the wellbore wall, which can adversely affect the bulk density measurement, two detectors, a “short-spaced” detector 200A and a “long-spaced” detector 200B, are employed such that their responses can be combined in a manner that minimizes the adverse effects in a way that is known in the art. The detectors 200A/B include substantially similar components to the detectors 200 in the tools 102A and 102B and are labeled with the same reference numbers. However, the casing 206A that surrounds the scintillation crystal 202 is selected to attenuate lower energy scattered gamma radiation that is most likely generated via photoelectric absorption (a different form of interaction than Compton scattering as described below). As illustrated, the source housing 304 and the detectors 200A/B are aligned within individual pockets 220 along a periphery of the tool 102C. As in the tool 102A, the drilling fluid conduit 210 is displaced from the central longitudinal axis of the detector tool 102C to accommodate the placement of the source 302 and detectors 200A/B, although other arrangements are also possible. Although the detectors 200A/B are illustrated symbolically with separate electronic circuitry 250, in actuality, the electronic circuitry 250 is common to both detectors 200A/B.
The scintillation crystal 202 in each of the above-described detectors 200 may be formed from materials that exhibit desirable scintillation properties upon interaction with gamma radiation such as sodium iodide doped with thallium (NaI(Tl)), cesium iodide doped with thallium or sodium (CsI(Tl) or CsI(Na)), bismuth germanate (BGO), or other organic or inorganic materials. The scintillation crystal 202 is encapsulated in a hermetically sealed casing 206 that is reflective to visible and ultraviolet light such that external light cannot enter the scintillation crystal 202 and light emitted by the scintillation crystal 202 in response to gamma radiation cannot escape. Light from the scintillation crystal 202 is optically coupled to the light sensor 204 via a transparent cover 208 (e.g., a glass cover) at one end of the scintillation crystal 202, as explained further below.
Gamma radiation refers generally to high-energy electromagnetic radiation having an energy level that exceeds 100,000 electron Volts (100 keV). Naturally-occurring gamma radiation within subterranean geological formations is primarily generated as a result of the radioactive decay of the naturally-occurring radioactive isotopes uranium-238 (Ur), thorium-232 (Th), and potassium-40 (K). Potassium decays directly to a stable element and emits a single gamma radiation photon in the process. Both thorium and uranium decay to stable elements through a chain of radioactive “daughter” elements and emit multiple gamma radiation photons in the process each having a unique energy. Spectra for these various elements are illustrated in FIG. 4, with each vertical line representing a probability of a gamma emission of a particular energy during radioactive decay of the specified element (and its daughter elements). As illustrated in FIG. 4, the energy of naturally-occurring gamma radiation measurable in a subterranean environment typically spans a range of about 0.1 to 3.0 million electron Volts (MeV). The energy of induced gamma radiation is generally between 0.05 and 0.8 MeV.
Gamma radiation detectors such as 200 identify the presence of gamma radiation through the interaction of the gamma radiation with the detector's scintillation crystal 202. Such interaction may occur through three primary mechanisms: photoelectric absorption, Compton scattering, and pair production. Photoelectric absorption involves the complete absorption of a gamma radiation photon by an atom in the scintillation crystal 202, which results in the emission of an electron having an energy that is equal to the energy of the photon minus the electron binding energy. Compton scattering involves a collision between the gamma radiation photon and an electron, which results in a transfer of energy from the photon to the electron. The decreased-energy photon may further interact with the scintillation crystal 202 through either another Compton event or through photoelectric absorption. Pair production involves the creation of a positron-electron pair from the energy of a gamma radiation photon in the vicinity of an atomic nucleus in the scintillation crystal 202. The likelihood of each of the three interactions is dependent upon the energy of the gamma radiation. Photoelectric absorption and Compton scattering are much more likely interactions than pair production at the energy levels of gamma radiation photons emitted by naturally-occurring radioactive materials and Compton scattering is the desired interaction mechanism for induced gamma radiation.
FIG. 5 illustrates further mechanical and electrical details of detector 200 and its electronic circuitry 250, and also describes the process through which gamma radiation photons are converted into measureable pulses. An incoming gamma radiation photon 270 traverses the light-reflective casing 206 (which is only reflective to lower-energy photons) and interacts with the scintillation crystal 202 through one of the interaction mechanisms described above. Regardless of the interaction mechanism, the resultant energetic electrons ultimately cause the emission of a larger number of lower-energy photons 272 (e.g., ultraviolet or visible light) as excited atoms in the crystal 202 return to the ground state. The lower-energy photons 272 have an energy content that is generally proportional to the energy of the gamma radiation photon 270.
Because the crystal 202 is surrounded by the light-reflective casing 206, the resulting photons 272 can only escape the crystal 202 through the transparent cover 208, which is optically coupled to the light sensor 204 (a photomultiplier tube, PMT) using an optical coupling fluid 212. Optical coupling fluid 212, such as a non-aqueous oil or grease, preferably has an index of refraction that very nearly matches that of the crystal 202 and the cover 208 to promote the efficient transfer of the photons 272 to the PMT 204.
The PMT 204 is an evacuated tube containing a photocathode 230, several dynodes 232, and an anode 234 within a glass housing. The photocathode 230 is held at a high negative voltage VPC that is supplied from a power supply 260, and the anode 234 is held at voltage VA which may be near a ground potential. The dynodes 232 form a voltage gradient between VPC and VA with each of the dynodes 232 being held at a higher potential than the node preceding it (i.e., VPC<VF<VE . . . <VB<VA). The voltage differential between VPC and VA may be on the order of 900 to 1100 Volts. The distributed voltages are created by a voltage divider circuit 233 including a number of resistors, which may be of equal value. It should be noted that the actual PMT 204 might include more dynodes 232 than shown in FIG. 5. Although not illustrated, electronic circuitry 250 may include one or more capacitors across the dynodes 232 to help stabilize their voltages as electrons propagate through the PMT 204, and as now explained.
As the photons 272 exit the crystal 202 through the cover 208, they strike the photocathode 230, which is a conductive, photosensitive coating that is applied to the surface of the PMT adjacent to the cover 208. Upon the arrival of the photons 272, a small group of primary electrons 274 is ejected from the surface of the photocathode 230 as a consequence of the photoelectric effect. The primary electrons 274 leave the photocathode 230 with an energy equal to the incoming photons 272 less the work function of the photocathode 230. Due to the geometric arrangement of the dynodes 232 and the manner in which they are biased, the primary electrons 274 are accelerated toward the first dynode 232F, increasing their kinetic energy. Upon striking the dynode 232F, the increased-energy primary electrons 274 cause the emission of a greater number of lower-energy secondary electrons, which are in turn accelerated toward the next dynode 232E. This process continues and results in an exponential increase in the number of electrons that arrive at the anode 234. For example, if at each dynode 232 an average of five new electrons are produced for each incoming electron, and if there are ten dynode stages, then each primary electron 274 will result in 510≈107 electrons arriving at the anode 234.
The large number of electrons arriving at the anode 234 produces an electrical pulse, which is measured by the electronic circuitry 250. Voltage-divider resistors 236 are sized to decrease the voltage at the anode 234 containing the pulse. A DC-blocking capacitor 240 removes the DC component of this decreased voltage signal, such that only the AC portion of the voltage signal caused by the pulse (and ultimately by the gamma radiation photon 270) is passed for further processing. This AC voltage signal is preferably pre-processed by a preamplifier 242 and further amplified by an amplifier 244. The amplifier 244 may further shape the electrical signal, for example, by generating a biphasic pulse with a shortened decay time.
FIG. 6A illustrates a stream of pulses 604 generated by the amplifier 244 in response to the detection of various gamma radiation photons 270 by the detector 200. As a result of the detection physics involved, the magnitude 606 of each pulse 604 is proportional to the energy of its associated gamma radiation photon 270. The pulses 604 are digitized by an Analog-to-Digital converter (ADC) 252 (FIG. 5), and the digitized pulses, or at least their magnitudes 606, are stored in a memory 254.
The digitized magnitudes 606 for each pulse are provided from memory 254 to a controller 256 (e.g., a microprocessor, a microcontroller, a FPGA, or other logic circuitry), which creates a gamma spectrum such as spectrum 610 or spectrum 620 as shown in FIG. 6B. Spectrum 610 is an example of a naturally-occurring gamma radiation spectrum and spectrum 620 is an example of an induced gamma radiation spectrum. Essentially, each of the gamma spectra 610 and 620 comprises a histogram, in which each pulse increments a count of a particular bin or “channel” based on its magnitude 606. In this regard, each of the channels is generally indicative of the energy of the incoming gamma radiation photon 270.
For example, in FIGS. 6A and 6B, the expected energy range of the incoming gamma radiation photons 270 (0 to 3.0 MeV) is split into 256 channels each spanning an energy range of approximately 11.7 keV. A count for a channel is incremented when a pulse 604 has a magnitude 606 that corresponds to that channel's magnitude range. In FIG. 6A, pulses 604A and 604D have magnitudes 606A and 606D that are both within a range associated with channel 22 (or an energy of about 257.4-269.1 keV); pulse 604B has a magnitude 606B within a range associated with channel 26 (or an energy of about 304.2-315.9 keV); pulse 604C has a magnitude 606C within a range associated with channel 17 (or an energy of about 198.9-210.6 keV); and pulse 604E has a magnitude 606E within a range associated with channel 67 (or an energy of about 783.9-795.6 keV). Given these example pulses, an associated gamma spectrum would have a count value of two for channel 22; and count values of one for each of channels 17, 26, and 67. Over time and given the occurrence of more pulses, the controller 256 will populate the entire gamma spectrum as shown in FIG. 6B. Note that count values for each channel may also be established by the controller 256 on a per-time basis, such that each channel is graphed versus a count rate (e.g., counts per second). “Count” as used herein should be understood as indicating either an absolute number of counts or a count rate. While the remainder of the specification refers generally to detectors measuring naturally-occurring gamma radiation and thus producing spectra such as spectrum 610, the invention described herein is equally applicable to detectors measuring induced gamma radiation and thus producing spectra such as spectrum 620.
Ideally, the shape of the gamma spectrum 610 depicted in FIG. 6B should generally represent the summed effect of all radioactive isotopes present in the formation 101. Thus, if only uranium is present in the formation, the gamma spectrum 610 should ideally bear resemblance to the uranium spectrum of FIG. 4. However, gamma spectra 610 as produced by detector 200 instead generally appear as shown in FIG. 6B as a smooth curve that peaks at lower energies. Such shape results from various non-idealities in detection, as well as the fact that detection is generally of more than one nuclear isotope. For example, the size of detector's scintillation crystal 202 affects the shape of gamma spectra 610 produced, because the crystal's ability to interact with higher-energy gamma radiation decreases as its size decreases. Thus, gamma spectra 610 typically exhibit a peak at the lower energies the scintillation crystal 202 is more easily able to detect, and the energy of such peaks will decrease as the size of the crystal decreases. Even given such non-idealities in the shape of gamma spectra such as 610, it is worth mentioning that such spectra may also provide details about the relative mixtures of radioactive isotopes to enable techniques such as clay typing and geochemical logging.
Once gamma spectra 610 are determined by the controller 256 and stored as a function of time/depth, they may be stored in memory 254 and later consulted via a computer system 114 (FIG. 1) once the drill string 106 and detector tool 102/detector 200 are retrieved from the wellbore 110 to better understand the composition of the formation. Alternatively, memory 254 may simply include the raw pulse magnitude 606 information stored as a function of depth/time, leaving it to the computer system 114 to generate gamma spectra 610 as a function of time/depth. Alternatively, and although not shown, electronic circuitry 250 can include means for communicating either raw pulse magnitude 606 information or gamma spectra 610 to the computer system 114, such as by mud pulse telemetry or other wired or non-wired means.
While gamma spectra 610 data can provide valuable information regarding the possible composition of the formation 101 traversed by the wellbore 110 as a function of depth, such data is most valuable when normalized to account for parameters that cause pulse magnitude 606 to vary, in particular temperature and time of operation of the PMT 204. FIG. 7 illustrates the effect of these two parameters on pulse magnitude 606. As illustrated, increased temperatures for prolonged operating times will cause decreases in pulse magnitude 606 for the same gamma event (e.g., a Potassium-40 emission). Such a change in pulse magnitude 606 can be referred to as a change in “gain” of the detector 200, which gain value is typically referenced to a value of 1.0, with decreases in gain comprising a value between 0 and 1.0, and increases in gain comprising a value of greater than 1.0. Unless the detector 200 is adjusted to accommodate for changes in gain these parameters cause, the pulse magnitudes 606, and hence the resulting gamma spectra 610 determined at various wellbore 110 depths, will not be comparable and thus will not accurately illuminate different possible resources at different portions of the formation.
Various prior art approaches focus on adjusting the gain of the detector 200 (i.e., the pulse magnitudes 606) by adjusting the voltage applied to the photocathode 230 (VPC) by the adjustable power supply 260. In this regard, note that the power supply 260 provides an adjustable regulated voltage output responsive to a voltage set point supplied by the controller 256, which may be delivered to the power supply 260 as an analog signal or via one or more digital signals. Such approaches varying VPC seek to normalize changes in gain caused by either or both of the temperature and operating time parameters, and generally fall into three categories.
A first category involves providing the detector 200 with an additional known low-level radiation source (sometimes known as an Energy Compensation Source, or ECS). Such a reference radiation source provides known gamma excitation to the detector 200, and thus VPC can be adjusted by the controller 256 to produce pulse magnitudes 606/gamma spectra 610 that are expected for this source. However, such prior art techniques are not preferred as they require accommodation of the additional reference radiation source with the tool 102, adding additional cost, safety, and regulatory considerations. Moreover, typical reference sources emit radiation at low energy levels at which it may be difficult to distinguish the signal corresponding to the reference source from signal noise caused by tool vibration that is prevalent in the low energy region.
A second category involves adding a temperature sensor with or near the detector 200, and adjustment of VPC based upon the measured temperature in conjunction with a temperature-VPC relationship table stored in the electronic circuitry 250. But this approach is not preferred because it also requires additional cost and complexity. Further, mere adjustment of VPC on the basis of temperature does not address the reality that operating time at temperature also causes variation, which requires changes to the temperature table to reflect the degradation in the performance of the PMT, as FIG. 7 shows. Further, such temperature sensing techniques are also subject to hysteresis, further compromising their accuracy over time.
A third category—into which the present invention also falls-involves algorithmic analysis by the controller 256 of measured gamma spectra 610, and resulting adjustment of VPC. For example, known gain stabilization techniques adjust photocathode voltage as a function of properties related to the location of a spectral peak. However, the spectral peak is often difficult to locate, especially in “clean” zones having lower radioactive material content and therefore lower count rates.
The invention disclosed in this application provides a new and improved algorithmic gamma spectral analysis technique for adjusting detector gain to compensate for variations in both temperature and operating life of the detector 200.